Conducting Briquet Technique of Spectrographic Analysis Analysis of Zirconium Metal as the Oxide VELMER A. FASSEL, ADDISOX 31. HOWARD', AND DARLENE ANDERSON* Institute for Atomic Research and Department of Chemistry, Iowa State College, Ames, Iowa Zirconium is a typical member of a group of new and industrially important metals which cannot be conveniently employed as metal self-electrodes for spectrographic analysis. Although zirconium metal can be readily converted to the oxide and excited in a conventional direct current carbon arc, experimental results have shown that the highly refractory oxide cannot be vaporized and excited in a reproducible manner. Moreover, acceptable internal standardization cannot be achieved with zirconium lines because of the selective volatilization mechanism. In searching for an approach to overcome these basic difficulties, the application of the conducting briquet excitation technique was investi-
A
KALYTIC$L emission spectroscopy has achieved its greatest success in the general area of metal and alloy analyses, especially when conducting electrodes of these materials have been excited under controlled excitation conditions. Unfortunately, the physical and chemical properties of many of the new, industrially important metals and alloys are such that homogeneous self-electrodes suitable for spectrographic excitation cannot be readily fabricated. Even if massive electrodes of uniform composition can be formed, or if conducting briquets can be fashioned from powders, millings, or drillings (20, 2.2, 26), one is still confronted with the problem of obtaining reliable standard samples in either massive or comminuted form. The conversion of these metal systems into a chemical form for which precise, reliable, synthetic standards can be prepared by conventional chemical methods offers a useful solution to this problem. Two standard conversion reactions-dissolution in acid solvents or ignition in air to form the corresponding oxides-are of general application. The solutions or solid oxides obtained from these procedures may then be excited directly by basic spectrographic excitation techniques, The logical initial step in the development of spectrographic methods for metal systems which cannot be excited as self-electrodes for the reasons outlined above is the selection of the most suitable conversion and excitation procedure. Primary emphasis should be placed on the simplicity and adaptability of the conversion procedure to routine operations, and amenability of the final sample form to reproducible spectrographic excitation. The subject matter of this communication is a description of this general approach as applied to the development of a method foi the determination of the common impurities found in zirconium metal.
gated. Unidirectional, interrupted, high energy, arclike discharges achieved uniform and reproducible volatilization of the zirconium oxide particles blended into the conducting powdered graphite matrix. Negligible selective volatilization was observed. Details are presented for the determination of aluminum, calcium, chromium, copper, iron, nickel, magnesium, silicon, and titanium in zirconium oxide with a coefficient of variation ranging from 3.1 to 6.9% for the various impurities. Because this technique makes i t possible to excite refractory samples with controlled excitation sources, it should find increasing application in spectrographic methods for the analysis of nonconducting substances.
the excitation stands. In contrast, zirconium metal turnings, millings, or shavings can be converted to the oxide through simple ignition in muffle furnaces a t 750" C. for 2 to 3 hours. From the standpoint of simplicity and convenience, this conversion procedure is preferable. Comparison of Excitation Techniques for Zirconium Oxide. The direct current arc between graphite supporting electrodes offers the simplest and most direct excitation technique for the analysis of nonmetallic powders ( 1 ). However, several investigators have reported that zirconium oxide cannot be vaporized in a reproducible manner, even in high current direct current arc discharges (1, 7 , 8, 2 4 ) . This is not surprising because zirconium oxide is one of the most refractory oxides known. Bnother difficulty associated with the refractory nature of zirconium oxide is that acceptable internal standardization cannot be achieved 11-ith zirconium lines, since the selective volatilization mechanism inherent in direct current arc vaporization methods causes most of the impurities to be virtually completely volatilized before the zirconium appears in the arc. This is shown in Figure 1, 5.0
n
EXPERIMENTAL
Conversion Procedures for Zirconium Metal. Zirconium metal is readily soluble only in hydrofluoric acid or mixtures of hydrofluoric and other acids. Although the dissolution of zirconium in hydrofluoric acid presents no basic difficulties, several factors detract from its simplicity. Glassware or quartz cannot be used in any of the operations if silicon is to be determined, and the preparation of hydrofluoric acid free of silicon and other impurities is not a simple task. I n addition, the direct excitation of the hydrofluoric acid solutions gives rise to troublesome corrosion in
TI ME (SECONDS)
Figure 1. Moving Plate Study of Spectral Line Intensity Variations during Direct Current Carbon Arc Excitation of Zirconium Oxide
1 Present address, Allis-Chalmers Manufacturing Co., Terre Haute Works, Terre Haute, Ind. 2 Present address, 319 Finkbine Park, Iowa City, Iowa.
Somewhat idealized Arcing current, 10 amperes
760
V O L U M E 25, NO. 5, M A Y 1 9 5 3
761
Reproducibility of Analytical Results for Carbon Arc Excitation Methods
Table I.
Determination Ca in ZrOz C a in ZrOz (a.c. arc) Si in ZrOl Cu in ZrOz T i in ZrOz
Average Deviation, 27-30
%
Observed by Ruehle and Jaycox (25) Ruehle a n d Jaycox (23) Fassel, Howard, a n d Anderson Fassel, Howard, a n d Anderson Fassel, Howard, and Anderson
14-15 10 16 4
